As a resistance identification art of the position sensor-less vector control system, in Japanese Patent Laid-open No. 2003-164188, before starting operation of a motor, a method for applying an inverter voltage so as to restrict the motor and identifying the resistance from the voltage and restriction current flowing and during operation, a method for installing a means for identifying the counter electromotive voltage factor of the motor, inferring the motor temperature from the value identified during operation and a preset counter electromotive voltage factor at normal temperature, and identifying the resistance using the inferred temperature are described.